OK...here is a part that I am making. Here is the mould. I did it with the EC gelcoat and paste for high temperature, anticipating that I could then use it for hand layup and prepreg. I am making my first part by hand layup (because I need to check all dimensions are good at the end). But man it was a mess with all the shapes...do you guys think that I would really be better with prepreg or would it be just as hard ?
Thanks !

I agree with everybody who says don't try it in one piece!  Hanaldo's idea of using a wet-preg method with plastic on both sides is what I'd do - especially if you don't have / haven't used pre-pregs.  It is a great way to really controlled wet-layup.  Infusion would be good too.

Here's a video (scroll down a bit in link) of the wet-preg method - not super helpful for a flat panel - but it's just to show the process: 
https://explorecomposites.com/materials-library/laminate-sample-15-vacuum-bagged-wet-layup-carbon-with-foam-core/

You'll need to pattern everything pretty well before you start the layup and with 1600-2000g of material (2mm) you have plenty of room for slip joints / laps at the joints with other plies cut back and butted - if it's really critical to have it all 2mm.  You may need to do a little grinding but it should come out well if you are careful about slip joints in the material (and bagging consumables) and leave plenty of bag in the inside corners.